The problem of fuel fires is ubiquitous for vehicles ranging from automobiles to jumbo jets and for fuel handling operations, particularly dispensing. In the field of civil aviation, over the last four decades, on average two air carrier accidents have occurred monthly within the US and fifteen worldwide. This level persists despite ongoing efforts to eliminate human error and improve security. Of these, an estimated 70% occur on takeoff or landing and are impact-survivable. It is further estimated that 40% of the fatalities in such crashes are due to fire caused by combustion of aviation fuel. Thus, it is estimated that some 500 to 1000 lives per year can be saved by the development of an effective mist control fuel system.
Perhaps more important is the issue of homeland security. The destructive power of a fully-fueled aircraft comes from the fuel—not kinetic energy. For example, in the case of the Sep. 11, 2001 attack on the World Trade Centers, both towers absorbed the aircraft's momentum and survived the initial impact. Threats to high-rise buildings, sports arenas, nuclear power facilities, and other important targets result from the explosion and intense post-crash fire. Thus, the successful incorporation of a mist control fuel additive would greatly reduce the property loss caused by plane accidents, and may serve to deter terrorists from using passenger aircraft fuel as a weapon of mass destruction.
Presently, there are no implemented technologies to reduce the fire hazard of fuel in crash scenarios. The associated safety and security issues are merely accepted as risks incident to air transportation. Attempts in the past to mitigate aircraft crash fires have led to the incorporation of firewalls, flame arresters, fuel-line isolation, fire extinguishing systems, fire-resistant materials, etc. with limited success. A 1997 NRC report points to this problem specifically, stating, “the reduction of the fire hazard of the fuel itself is critical in improving survivability in past crashes.” (See, e.g., NCR Proceedings, NMAB-490, Washington D.C., 1997, the disclosure of which is incorporated herein by reference.) Candidate technologies aimed to provide a post-impact fire-safe fuel were evaluated in the January 2000 report of Southwest Research Institute, prepared by Bernard Wright under contract to NASA. (See, e.g., “Assessment of Concepts and Research for Commercial Aviation Fire-Safe Fuel,” Bernard Wright, SWRI, January 2000, the disclosure of which is incorporated herein by reference.) Based on an extensive review of fuel vulnerability studies and discussions with industry-knowledgeable sources, Mr. Wright concluded that Mist-Controlled Kerosene (MCK) technology was rated as the most promising and highest priority to date.
MCK is a conventional Jet-A fuel to which a small fraction of high MW polymer (<0.3 wt %) has been added. When fuel is released from ruptured tanks into the airflow around a crashing aircraft, the polymer interferes mechanically with the formation of mist. Linear polymer chains that have high enough MWs are well known to be effective mist-control agents (i.e. of MW>˜5,000,000 g/mol, abbreviated as 5M g/mol). Unfortunately, ultralong homopolymers degrade upon passage through pumps, filters or long pipelines (termed “shear degradation”), rendering them ineffective for mist suppression. Attempts to create polymers that suppress misting and resist flow degradation have been made, primarily by adding randomly placed associating groups onto fuel-soluble polymers. Among the large number of associating polymers synthesized in an attempt to achieve mist-control the one that advanced the farthest technologically was “FM-9”, a proprietary polymer developed by the British company Imperial Chemical Industries (ICI) in the 1970's. The last major attempt at polymer-modified fuel was the FAA-funded anti-misting kerosene (AMK) program centered on FM-9, shown in FIG. 1a. By trial and error a fuel formulation was produced that reduced the post-crash fire; however, ultimately the program failed because the formulation could not be produced, transported and stored using existing fuel handling systems. It did not have adequate resistance to shear degradation, so it could not be pumped or filtered (therefore, the additive could not be incorporated at the point of production of the fuel or in any centralized facility). Efforts to add the polymers to the fuel at the point of delivery have proven impractical both economically and from a regulatory point of view. Furthermore, FM-9 tended to phase separate, particularly under cold conditions, so it could not be stored or used at low temperature (e.g., it would deposit on the walls of storage tanks and would clog fuel filters). The FM-9 polymers lost efficacy under hot conditions due to disruption of the attraction between associating groups.
Several groups have found experimentally that by placing associative groups randomly along polymer chains, the molecules could be made to ‘stick’ to each other in aggregates of approximately 10 to 100 chains. Therefore, aggregates greater than 5M g/mol can be achieved using polymers of MW 0.1M-1M g/mol that are small enough that they do not undergo shear degradation during fuel handling. Some experiments indicated that flow degradation could be reduced using randomly placed associative groups (because aggregates could pull apart during brief intervals of intense flow and reassemble again afterward.) Nevertheless, despite the extensive effort devoted to randomly functionalized associative polymers, no commercially viable mist control fuel has yet been produced. The polymers were not effective at the dilute conditions of most interest for fuel additives (for example, FM-9 was used at 0.3% wt, a much greater concentration than any other fuel additive). At dilute concentrations, the associating groups within a given polymer drive the polymer to collapse upon itself, rendering it ineffective for mist suppression. Furthermore, addition of associating groups to a fuel-soluble polymers tends to drastically reduce its solubility in fuel, leading to phase separation, which leads to unacceptable behavior during storage and use (noted above in relation to FM-9)
In an attempt to remedy the loss of multi-chain clusters at dilute concentrations, a few studies have examined “donor-acceptor” associative polymers that use two different polymers, one bearing randomly placed “donor” groups (that do not associate with each other) and the other polymer bearing randomly placed “acceptor” groups (that do not associate with each other). The driving force for association causes “donor chains” to associate with “acceptor chains”, even under dilute conditions. Unfortunately, as will be fully developed below, all of these prior polymer design concepts aimed at reducing the fire hazard of fuel were misguided. Even supramolecules held together by association of randomly distributed donor and acceptor groups exhibit chain collapse under dilute conditions: the multi-chain aggregates are densely “stuck” to one another and occupy a much smaller volume than the unfunctionalized, separate chains would. Despite the high molar mass of the aggregate, they are less effective for mist suppression than the corresponding homopolymers (with no associative groups at all).
Accordingly, despite decades of effort, no polymer design has been discovered that can overcome shear degradation and avoid chain collapse, and thereby provide effective mist control. The current invention describes mist control polymers that have the following properties:                Can be added at the refinery where other fuel additives e.g., anti-static agents, are introduced;        Provides effective fire protection at low concentrations between 50-500 ppm;        Withstands unintentional degradation during fuel handling;        Does not deposit onto materials used in storage tanks, filters and transfer systems;        Will be compatible with current aviation fuel handling and pumping systems; and        May be synthesized at an acceptable cost.        